This is an application with three specific aims. In the first specific aim the Principal Investigator will assess quantitative viral load in plasma and CSF over time, and derive estimates of cumulative viral load. These will be correlated with measures of CNS dysfunction. In the second specific aim, the Investigator will use an assay of neurotoxicity based on cultured cells and relate the cytotoxicity of CSF from HIV seropositive individuals to viral load. In the third specific aim, the relationship between cytotoxicity and neurological disease will be measured.